There has recently been a marked increase in the production of crystalline silicon solar cells which utilize plates of monocrystalline or polycrystalline silicon as substrates. Such solar cells have electrodes for output of the generated power.
As an example, a schematic cross sectional view of a crystalline silicon solar cell is illustrated in FIG. 1. In a crystalline silicon solar cell, an n-type diffusion layer (an n-type silicon layer) 3 is generally formed on the light incident side of a p-type crystalline silicon substrate 4. An antireflection film 2 is formed on the n-type diffusion layer 3. Further, a light incident-side electrode 1 is formed by a series of steps in which a conductive paste is applied by a method such as screen printing to print a pattern corresponding to the light incident-side electrode 1 (the front electrode) on the antireflection film 2, and the conductive paste is dried and fired. During firing, the conductive paste fires through the antireflection film 2 with the result that the light incident-side electrode 1 comes to be in contact with then-type diffusion layer 3. Here, fire through is the etching of the insulating antireflection film with materials such as glass frits present in the conductive paste so as to conductively connect the light incident-side electrode 1 and the n-type diffusion layer 3. Because light does not have to come through the backside of the p-type silicon substrate 4, a back electrode 5 is generally formed substantially over the backside entirely. The p-type silicon substrate 4 and the n-type diffusion layer 3 form a p-n junction at their interface. Light such as sunlight passes through the antireflection film 2 and the n-type diffusion layer 3 and enters into the p-type silicon substrate 4. The light is absorbed during this process, generating electron-hole pairs. The electric field at the p-n junction separates the electron-hole pairs, the electrons being attracted to the light incident-side electrode 1 and the holes to the back electrode 5. The electrons and the holes are output as electric current through these electrodes.
Conventionally, for the formation of electrodes of solar cells, in particular crystalline silicon solar cells, conductive pastes which contain conductive powders, glass frits, organic binders, solvents and other additives, are used. A silver powder is mainly used as the conductive powder.
For example, Patent Publication 1 (JP 2009-193993 A) discloses a process for manufacturing a solar cell electrode wherein the electrode is formed on a main surface of a silicon substrate of a solar cell and a wire for output of the power generated in the silicon substrate is connected to an upper portion of the electrode via a solder, the process comprising a step (a) of printing a first silver paste which includes a glass frit containing lead oxide and bismuth oxide onto the main surface of the silicon substrate, a step (b) of printing a second silver paste onto the first silver paste, the second silver paste including a glass frit having lower contents of lead oxide and bismuth oxide than in the glass frit of the first silver paste, and a step (e) of firing the first silver paste and the second silver paste.
Patent Publication 2 (JP 2008-109016 A) discloses a silver paste for solar cell elements comprising a glass frit having a composition containing, in terms of oxide, 5 wt % to 10 wt % of zinc oxide, 70 wt % to 84 wt % of bismuth oxide, and 6 wt % or more of boron oxide and silicon oxide combined, as well as a silver powder and an organic vehicle.
Patent Publication 3 (JP 2011-503772 A) discloses a silicon semiconductor device, and a conductive silver paste for use in the front side of a solar cell device. In detail, Patent Publication 3 discloses a thick film composition comprising a) an electrically conductive silver powder; b) one or more glass frits; and c) (a) Mg and (b) a specific Mg-containing additive, d) dispersed in an organic medium. Patent Publication 3 describes that the glass frit comprises Bi2O3 and B2O3 at 8 to 25 wt % of the glass frit, and further comprises one or more components selected from the group consisting of SiO2, P2O5, GeO2 and V2O5.
Patent Publication 4 (JP 2011-502345 A) and Patent Publication 5 (JP 2011-502330 A) disclose thick film compositions comprising glass frits similar to those of Patent Publication 3.
Patent Publication 6 (JP 2011-171272 A) discloses an electrode paste composition comprising copper-containing particles exhibiting the largest exothermic peak at a peak top temperature of 280° C. or more according to thermogravimetric and differential thermal analysis, glass particles, a solvent and a resin. Patent Publication 6 describes that the glass particles have a glass softening point of 600° C. or less and a crystallization onset temperature exceeding 600° C.
Patent Publication 7 (WO 2009/139222 A) discloses a solar cell comprising a semiconductor substrate with a p-n junction, and a silver electrode and an aluminum electrode on the backside of the semiconductor substrate, the silver electrode and the aluminum electrode overlapping with each other at portions, the silver electrode containing a glass component having a glass softening temperature equal to or higher than the glass softening temperature of a glass component in the aluminum electrode.
Patent Publication 8 (JP 2011-066134 A) discloses a silver paste for forming solar cell silver electrodes which comprises silver particles, a polyvinyl acetal resin, a glass frit and an organic solvent, the content of the silver particles being 70 to 95 wt %, the content of the polyvinyl acetal resin being 0.1 to 5 wt %, the content of the glass frit being 0.1 to 5 wt %. Patent Publication 8 describes that a lead borosilieate glass frit having a softening temperature of 300° C. or more and not more than the firing temperature (for example, 800° C.) is used as the glass frit. Patent Publication 8 also describes that the lower and upper limits of the glass frit content are 0.1 wt % and 5 wt %, respectively, relative to the paste.
Patent Publication 9 (JP 2010-263136 A) discloses an electrode conductively connected to a semiconductor substrate that is a sintered body comprising silver particles and a glass frit, the silver particles containing 50 to 80 mass % of particles with an average particle diameter of 10 μm or more. Patent Publication 9 describes that the sintered body may be obtained by sintering a silver paste including the silver particles, the glass frit, an organic vehicle and an organic solvent. Patent Publication 9 also describes that this silver paste preferably contains 55 to 90 mass %, particularly 75 to 85 mass % of the silver particles, and 0.2 to 5 mass %, particularly 0.5 to 2 mass % of the glass frit.
Patent Publication 10 (JP 2009-231826 A) discloses a conductive composition comprising a silver powder, a glass powder containing PbO, and an organic vehicle, the composition being used to form an electrode conductively connected to an n-type semiconductor layer through a silicon nitride layer, the n-type semiconductor layer being disposed under the silicon nitride layer, the content of the silver powder in the composition being 70 mass % to 95 mass %, the content of the glass powder being 1 to 10 parts by mass with respect to 100 parts by mass of the silver powder, the glass powder having a basicity of 0.6 to 0.8 and a glass transition temperature of 300° C. to 450° C.    Patent Publication 1: JP 2009-193993 A    Patent Publication 2: JP 2008-109016 A    Patent Publication 3: JP 2011-503772 A    Patent Publication 4: JP 2011-502345 A    Patent Publication 5: JP 2011-502330 A    Patent Publication 6: JP 2011-171272 A    Patent Publication 7: WO 2009/139222 A    Patent Publication 8: JP 2011-066134 A    Patent Publication 9: JP 2010-263136 A    Patent Publication 10: JP 2009-231826 A